Here is what Mercedes-Benz says about it:

The Brake Assist System (BAS), an electronic system by Mercedes-Benz, interprets the braking behavior of the driver and initiates the full braking effect when it identifies an emergency situation. This reduces the braking distance substantially.

Here is the engineering foundation for BAS:

Conventional braking systems usually use engine vacuum to increase braking capability. Instead of relying solely on vacuum power to provide effective brakes, an electric pump pressurizes brake fluid to provide power assist for BAS and the necessary brake force for 4ETS.

The pressurized brake fluid is stored at about. 180 bar (2600 psi!) in a reservoir known as the accumulator. From the accumulator the high pressure brake fluid is supplied to the hydraulic operating cylinder in the master cylinder assembly. Commands to utilize the pressurized fluid comes from either 4ETS or BAS. When the brakes are applied and BAS deems it necessary, a lever connected to the brake pedal releases varying amounts of the high pressure fluid to assist in stopping the car.

Here is what BAS does for you:

Most drivers, under normal braking conditions as well as under emergency conditions, start out with little brake pressure and whenever necessary they will increase their pedal effort. In an emergency this behavior leads many times to a crash since the car could not be stopped in time. Those situations require maximum pedal pressure from the beginning - if necessary the effort can be reduced later in the process.

With other words most drivers do not use the ability of the brakes to their advantage - BAS automatically corrects that. The system recognizes emergency situations within milli seconds and releases pressurized brake fluid into the system as soon as the driver touches the brake pedal. As soon as the driver releases the brake pedal, BAS kicks back into a standby mode.

BAS creates a much higher stopping force for emergencies than most drivers are ever able to generate. By doing so BAS helps to prevent accidents
 

Just another good reason to drive a car made by Mercedes-Benz

Taking steering a step further: ESP

Using ABS sensors for traction control: 4ETS